simplify travis caching

.travis.yml

@@ -2,37 +2,15 @@ sudo: no
 
 language: python
 
-cache:
-  # Apparently if you override the install command that silently disables the
-  # cache: pip support. This is less than ideal and I've opened up
-  # travis-ci/travis-ci#3239 to hopefully get that addressed. For now I'll
-  # manually add the pip cache directory to the build cache.
-  directories:
-    - ~/.cache/pip
-
-env:
-  global:
-    # These two environment variables could be set by Travis itself, or Travis
-    # could configure itself in /etc/, ~/, or inside of the virtual
-    # environments. In any case if these two values get configured then end
-    # users only need to enable the pip cache and manually run pip wheel before
-    # running pip install.
-    - PIP_WHEEL_DIR=$HOME/.cache/pip/wheels
-    - PIP_FIND_LINKS=file://$HOME/.cache/pip/wheels
+cache: pip
 
 python:
 - '2.7'
-- '3.3'
 - '3.4'
+- '3.5'
 
 install:
-  # Before installation, we'll run ``pip wheel``, this will build wheels for
-  # anything that doesn't already have one on PyPI.
-  - pip wheel -r requirements.txt
-  # Actually install our dependencies now, this will pull from the directory
-  # that the first command placed the Wheels into.
   - pip install -r requirements.txt
-  # we also need to install the test dependencies
   - python setup.py install
 
 script: